Once again, that thing looked like an imitation of a suit-case bomb, Engineering teacher implied exactly that when he said "Don't show it to anyone"
And if someone does not see that then he is probably living in a cave without any exposure to TV.
I don't see anything wrong with school banning items which may look like an actual weapon.
The only question in tis story is question of intent, and so far it seems intent was to impress his teacher with this stupid "clock in a case" thing. The keyword here is "stupid", because it was in fact stupid and when people do stupid things other people often get confused, and that's what happened here.
Ever built a homemade electronic device?
If you do, you're going to have to put it all in some kind of container.
If you intend to bring it from home to school, it makes sense to build it in a case that can be used for transport.
This is not significantly different from countless homemade devices made by countless amateur engineers, yet strangely most of them don't get arrested.
Besides, you are completely ignoring the fact that none of the teachers nor cops behaved at all in the way one would behave if they believed someone brought a bomb to school. They did not attempt to evacuate the school. They did not call the bomb squad. They kept the student and the "bomb" in an office with administrators, then put the student and the "bomb" in a squad car with a police officer. That simply isn't what people do when they believe there is a bomb at a school full of children.
So no. It is
not reasonable to assume that it was a bomb, and none of the teachers nor cops actually thought that it
was a bomb.